cDNA cloning of the human retinal rod Na-Ca + K exchanger: comparison with a revised bovine sequence.
OBJECTIVE
To clone the complementary DNA of the human retinal rod Na-Ca + K exchanger.


METHODS
A human retinal cDNA library was screened initially with a radiolabeled probe representing the entire bovine rod Na-Ca + K exchanger cDNA and subsequently with probes from polymerase chain reaction fragments of the human retinal rod Na-Ca + K exchanger obtained after the initial screen. Twelve positive clones were used to obtain the entire coding sequence of the human retinal rod Na-Ca + K exchanger.


RESULTS
The cDNA of the human retinal rod Na-Ca + K exchanger codes for a protein of 1081 amino acids, which shows 64.3% overall identity with the bovine retinal rod Na-Ca + K exchanger at the amino acid level. The two sets of putative transmembrane-spanning domains and their short connecting loops showed the highest degree of identity (94%-95%), whereas the extracellular loop at the N terminus showed a 59% identity. The large cytosolic loop that bisects the two sets of transmembrane-spanning domains contained two large deletions in the human exchanger; the first deletion contains 18 amino acids, whereas the second deletion involves a series of repeats that are dominated by acidic amino acid residues observed in the bovine, but not in the human, sequence. The authors observed that the bovine sequence contains a ninth repeat in addition to the eight repeats of the published sequence.


CONCLUSIONS
The authors cloned the cDNA of the human retinal rod Na-Ca + K exchanger as a first step in examining the possibility that this gene could be the locus of disease-causing mutations.
Tucker
J E
JE
Department of Medical Biochemistry, Faculty of Medicine, University of Calgary, Canada.
Winkfein
R J
RJ
Cooper
C B
CB
Schnetkamp
P P
PP
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002020:Buffaloes; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism; D002417:Cattle; D003001:Cloning, Molecular; D017931:DNA Primers / Q000737:chemistry; D018076:DNA, Complementary / Q000032:analysis*; D005136:Eye Proteins / Q000235:genetics* / Q000378:metabolism; D006801:Humans; D008969:Molecular Sequence Data; D011188:Potassium / Q000378:metabolism; D012374:Rod Cell Outer Segment / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D012964:Sodium / Q000378:metabolism; D019831:Sodium-Calcium Exchanger
D002352:Carrier Proteins; D017931:DNA Primers; D018076:DNA, Complementary; D005136:Eye Proteins; D019831:Sodium-Calcium Exchanger; C074530:potassium-dependent sodium-calcium exchanger; D012964:Sodium; D011188:Potassium; D002118:Calcium